Run sanity checks before/after installation

[Mark-Simulacrum]

Not sure if this belongs in rustup, but I think this is the best place for it today. We should try and detect if the user has the minimum requirements for Rust installed before finishing installation. These include a linker (cc) and possibly more -- I don't know.

[nodakai]

Once #313 is successfully resolved, there can be use cases where a privileged sysadmin without cc in $PATH wants to install Rust with rustup to somewhere like /opt/rust. (It is uncommon to see environments such as university workstations where working C compilers exist only under /opt rather than in everyone's $PATH by default and programmers are supposed to specifically enable them with their .profile .)

So please don't make the check for cc mandatory.